Thin film EL devices provided with electroluminescent (hereinafter referred to as EL) panels have features such as high visibility, excellent display capability, and high speed response.
Of the thin film EL devices, on, for example, organic EL devices utilizing organic compounds as the constituent material, various reports have been made. Among these reports, for example, C. W. Tang et al. proposed an organic electroluminescent device (organic EL device) having an organic luminescent layer and a hole-transporting layer stacked on top of each other (Applied Physics Letters, 51, 1987, p. 913). Up until today, the research and development of the thin film EL device have been conducted based on this organic electroluminescent device having a basic configuration.
The basic configuration of the organic electroluminescent device will be described below.
FIG. 16 is a schematic cross-sectional view showing a prior art organic electroluminescent device. The organic electroluminescent device has, as shown in the figure, a transparent electrode 102, a hole-transport layer 103, an electron-transporting luminescent layer 104, and a cathode 105 stacked in sequence on a glass substrate 101. In another configuration, the electron-transporting luminescent layer 105 may be divided by functions into an electron-transport layer and a luminescent layer.
It should be noted that the thin film EL device is defined as a device in which functional layers interposed between the transparent electrode 102 and the cathode 105 are organic layers, inorganic layers, or mixed layers of an organic layer and an inorganic layer. The organic electroluminescent device is defined as a device in which the functional layers interposed between the transparent electrode 102 and the cathode 105 are organic layers.
Here, the cathode 105 utilizes an alloy composed of an alkali metal or an alkaline earth metal which has a low work function and a stable metal such as aluminum or silver, as the cathode with which, electron injection is performed stably and readily. Specific example of the cathode 105 made of the alloy is disclosed, for example, in Japanese Unexamined Patent Publication No. 5-121172. In this publication, it is described that an alloy composed of aluminum and lithium is utilized and the Li concentration is controlled in the trace range from 0.01 wt % to 0.1 wt %. Thereby, an organic electroluminescent device with high electroluminescent efficiency and high environmental stability can be realized.
In relation to the prior art described in the above-described publication, another organic EL device is disclosed which has, in place of the cathode 105, a metal thin film serving as an electron-injecting electrode and an electrode film serving as a passivating electrode stacked in sequence on the organic layer. The metal thin film is composed of a low work function metal material such as an alkali metal or an alkaline earth metal, and the electrode film is composed of a metal material which is stable to oxygen and water. With such a configuration, the electrode film can protect the metal thin film, which has a high reactivity with moisture and the like, from the outside, thereby maintaining electron injection efficiency. Furthermore, since it is not necessary to control Li and the like at low concentrations, device fabrication with simplified processes is made possible.
Moreover, in recent years, an organic electroluminescent device having, as shown in FIG. 17, an electron-injecting layer 106 provided between an electron-transporting luminescent layer 104 and a cathode 105 has been disclosed (Japanese Unexamined Patent Publication No. 9-17574). In this publication, it is disclosed that the electron-injecting layer 106 utilizes an alkali metal compound as the material, and by optimizing the film thickness of the electron-injecting layer 106, light emission with high luminance can be obtained. Furthermore, it is described that because alkali metal compounds are chemically stable, reproducibility of characteristics is high, making it possible to obtain an organic EL device capable of emitting light with high luminance at low applied voltages.
As for an electron-injecting layer composed of an insulating substance, the relationship between the film thickness and a dark spot (a region where there is no light emission) and the like are reported in detail (T. Wakimoto, Y Fukuda, K. Nagayama, A Yokoi, H. Nakada, and M. Tsuchida, IEEE Transactions on Electron Devices, Vol. 44, No. 8, p. 1245, 1997).
The present inventors have developed an organic electroluminescent device which utilizes, as the material for the electron-injecting layer, a specific organic metal complex compound containing an alkali metal or an alkaline earth metal as the central metal, or an electron-deficient compound. With an organic electroluminescent device having the electron-injecting layer utilizing such materials, an electroluminescent device with high luminance and long lifetime can be obtained without the need to use low work function metal materials.
As has been described, in developing the organic electroluminescent device, the electron-injecting layer is a key element in determining electroluminescent efficiencies and lifetimes. From this perspective, various improvements have been made.
When these organic electroluminescent devices are driven under duty drive conditions, for practical application, the instantaneous luminance reaches several thousands to several ten-thousands of cd/m2. Thus, when driving the organic electroluminescent device, it is necessary to maintain high efficiencies even in such a high luminance region. Hence, in prior art organic electroluminescent devices, the luminance needs to be further increased. To realize this, it is indispensable to use an electrode containing an alkali metal or an alkaline earth metal which has an excellent electron injection efficiency.
The electrode utilizing such a low work function metal material, however, may cause degradation in device characteristics due to deterioration of the metal. In order to prevent the degradation, as is described above, a method employing a configuration in which low work function metal compounds are utilized was attempted. With this method, when utilizing inorganic metal compounds, satisfactory improvement was not made in terms of efficiencies and lifetimes. On the other hand, when utilizing organic metal compounds, although some improvement was made in terms of degradation of device characteristics, for achieving further improvement, simple metals with low work function were required to be utilized as the electrode material.
Thus, when low work function simple metals with high reactivity are utilized as the electrode material, deterioration of the simple metals must be prevented.